1) Draw a circle of radius 20 cm and center C.
2) Draw line segment AB.
Construct the perpendicular bisector of AB.
The perpendicular bisector of AB will intersect the circle in two points; these points are the solution.
To bisect a line segment:
-- open you compass to a little more than half of the length of the line segment.
-- draw an arc from one of the endpoints through the line segment.
-- draw an arc from the other endpoint through the line segment.
-- there should be two points of intersection of the arcs, one on one side of the line segment;
the other on the other side.
-- connect these two points of intersection with a line
-- this line is the perpendicular bisector of the original line segment.
-- where this line crosses the original line segment is the midpoint of that line segment.
Qestion #2: [Remember that you have to scale this!]
1) Find the point X on DA such that DX = 2m.
Find the point Y on CB such that CY = 2m.
Connect X and Y to form line segment XY.
2) Construct a circle with center A and radius 4 m.
The solution is the point where line segment XY crosses the circle.